Nest predation and cowbird parasitism create a demographic sink in wetland-breeding song sparrows

We studied mechanisms underlying poor breeding performance in a color-banded population of Song Sparrows (Melospiza melodia) in coastal British Columbia for four years. Only 21% of sparrow nests produced fledglings. The main causes of nest failure were complete predation (42% of nests), and abandonment after parasitism by Brown-headed Cowbirds (Molothrus ater) coincided with partial predation (34%). Increasingly fewer sparrow young fledged as the number of cowbird eggs per nest increased, but nests with up to five cowbird eggs fledged cowbirds. Many nests produced only cowbird fledglings. Annual production of recruits was seven times below the annual rate of adult disappearance. Nevertheless, Song Sparrow numbers at the study site remained stable due to immigration. When reproductive success was estimated in the absence of cowbird parasitism, production of young remained poor. Thus, high levels of predation on nest contents explained the poor breeding performance. Experiments with dummy Song Sparrow eggs in Song Sparrow nests suggested that mice, shrews, and medium-sized birds were the principal egg predators, but these small mammals probably were not frequent predators on Song Sparrow eggs. Damage by bird predators coincided with the laying season of the cowbird, suggesting that cowbirds contributed to predation on sparrow nests. The study site is a population sink for Song Sparrows because of high rates of nest predation and brood parasitism. These ecological factors may be contributing to the significant long-term decline (1966-1994) of the Sang Sparrow in British Columbia and throughout Canada.